1. Field of the Invention
The present invention relates to a voltage generating apparatus, more particularly, to a voltage generating apparatus with a fine-tune current module.
2. Description of the Prior Art
Almost all analog or mixed-mode circuits need reference voltages to provide the bias voltage. The reference voltage can generate a constant and reproducible voltage even during process variation, change of ambient temperature, and supply voltage instability so that the circuits can operate with accurate DC bias. Therefore, a DC voltage generator is an important block in many circuits.
A well-known method of generating a stable reference voltage is to utilize the phenomenon of semiconductor bandgap in a reference circuit. The bandgap energy of a semiconductor will change predictably with ambient temperature, and bandgap reference circuits are designed according to this principle. The most popular method of generating bandgap voltage in the prior art is to connect the base and the collector of a BJT to form a diode-like structure, so the voltage difference (Vsub) between the base and the emitter of the BJT can be the bandgap voltage.
Please refer to FIG. 1. FIG. 1 illustrates temperature variation versus Vsub in a diode-like device. As shown in FIG. 1, Vsub linearly decreases with rising temperature. If one can generate another voltage (like the compensation voltage in FIG. 1) which linearly increases with rising temperature at the same rate as Vsub decreases, the summation of the two voltages results in a constant reference voltage that reduces variation due to temperature.
Please refer to FIG. 2. FIG. 2 illustrates a reference voltage generator 200 implementing the bandgap voltage principle. The reference voltage generator 200 is a feedback control system that maintains two inputs of the amplifier 230 at similar levels. In the reference voltage generator 200, the diodes D1 and D2 have different section areas corresponding to different current densities in order to adjust the slopes of the temperature coefficients of the two diodes, D1 and D2. When the voltage generator 200 is operating, the voltage difference of VD1 and VD2 (Vdel) expresses a characteristic of a positive temperature coefficient (a positive slope in the temperature function), but the voltage VD1 expresses a characteristic of a negative temperature coefficient, like the property of an ordinary semiconductor. Through the combination and arrangement of the diodes D1, D2 and the amplifier 230, the amplifier 230 will output a stable voltage regulated against temperature variation resulting from compensation of the voltage with the positive temperature coefficient, and the voltage with the negative temperature coefficient. However, in the modern IC industry, more mature CMOS technology achieves lower production costs. Thus, the reference voltage generator in FIG. 1 implemented by BJTs has the disadvantage of higher price compared to some products. Moreover, the bandgap of silicon, being about 1.2V to 1.3V, cannot satisfy future trends in low power applications.
Due to lower costs and more mature technology, a voltage generator of another prior art is implemented by MOSFETs. In this case, the voltage is generated by operating a MOS device in the sub-threshold region.
When a MOS device is operating in the sub-threshold region, if the device is given a fixed drain current, the voltage difference between the gate and the source of the device will linearly decrease with an increase of ambient temperature. In other words, the voltage difference shows a negative temperature coefficient in this situation. Please refer to FIG. 3; FIG. 3 illustrates a voltage generator 300 utilizing the negative temperature coefficient of a MOS device according to the prior art. The voltage generator 300 has two parts. The first part includes MOS MM1 to MOS MM4, and a resistor R1, wherein the MOS MM1 is designed to operate in the sub-threshold region and the current IRR1 through the resistor RR1 relates to the voltage difference between the gate and the source of the MOS MM1. The second part includes MOS MM5 to MOS MM11 and the resistors RR2, RR3 and RR4. The second part generates an output voltage VR by compensating the current IRR1 of a negative temperature coefficient and a current of a positive temperature coefficient. The voltage generating method not only has lower production costs but also can generate a lower reference voltage to provide a small voltage bias for low power circuits.
However, the prior art in FIG. 3 has the disadvantage that although the generated voltage is stable with respect to temperature variation, the actual voltage output of the circuit will deviate from the design value due to processing variation. Therefore, the voltage generators in the second prior art have different output voltages if implemented by different process corners.